A spark plug is used for ignition of an internal combustion engine such as an automobile engine. The spark plug generally includes a tubular metallic shell, a tubular insulator disposed in an inner hole of the metallic shell, a center electrode disposed in a front side inner hole of the insulator, and a ground electrode joined at one end thereof to the front side of the metallic shell with a spark discharge gap provided between the other end of the ground electrode and the center electrode. The spark plug causes spark discharge at the spark discharge gap formed between the front end of the center electrode and the front end of the ground electrode in a combustion chamber of an internal combustion engine, to burn fuel with which the combustion chamber is filled.
In recent years, the temperature in the combustion chamber tends to be enhanced for high output and improvement of fuel efficiency. Further, an engine in which a discharge portion forming the spark discharge gap is disposed so as to project into the combustion chamber in order to improve ignitability, has been brought into use. In such circumstances, since the discharge portion of the spark plug is subject to high temperature, oxidation wear tends to progress in the center electrode and the ground electrode that form the discharge portion. Therefore, a method in which tips are formed at the opposing front ends of the center electrode and the ground electrode, to cause spark discharge between the tips, thereby reducing the oxidation wear of the center electrode and the ground electrode (hereinafter, may be each referred to as an electrode), is being developed.
Further, attempt is made for achieving both high output and improvement of fuel efficiency by enhancement of a combustion pressure which is brought by engine downsizing and use of a direct injection turbo engine. In such an engine, a difference in temperature in the combustion chamber between in combustion and in intake charge, is increased, and a difference in temperature in the combustion chamber between in supercharging and in normal operation, is also increased. Thus, the spark plug tends to be placed in a severe thermal cycle environment. Therefore, a problem arises that the tip is likely to peel off. Further, a pressure in the combustion chamber is likely to be increased, and, according to the increase of the pressure in the combustion chamber, a spark discharge voltage is increased, so that a problem arises that the spark wear is likely to be caused at the tip. Therefore, not only the problem that oxidation wear due to high temperature is to be reduced but also the problem that peeling of the tip from the electrode due to the severe thermal cycle is to be reduced, and spark wear at the tip due to increase of a spark discharge voltage is to be reduced, need to be simultaneously overcome.
As a method for solving the problem, among the problems, that peeling of the tip from the electrode is to be reduced, a method is suggested in which a tip, having a flange portion, such as an electrode tip “having, at the one end of the electrode tip, a flange portion having a diameter larger than the diameter of the other end” (Japanese Patent No. 4015808) and a tip “formed by a flange portion and a projection that projects from one surface of the flange portion” (Japanese Patent Application Laid-Open (kokai) No. 2008-34393), is used.
JP 4015808 and JP 2008-34393 disclose that, for example, “an alloy that contains Ir as a main component and contains: “(1) 3 to 50% by mass of Rh, (2) 1 to 10% by mass of Pt, or (3) 50% by mass or less of Rh and one of Ru or Pt in total, and 1% by mass or more of Rh, 1% by mass or more of Ru, and 1% by mass or more of Pt”, is used as a material of the tip (see claim 5 of JP 4015808), and “a Pt alloy that contains at least one of 20 to 60% by mass of Rh, 10 to 40% by mass of Ir, and 1 to 20% by mass of Ni”, is used as a material of the tip (see paragraph 0022 of JP 2008-34393). Meanwhile, as described above, since a spark plug that can exhibit a desired performance under a severe environment, is required, use of a material of the tip which is still further excellent in oxidation resistance and spark wear resistance is desired. When a Pt—Rh based alloy is used as a material of a tip having a flange portion, both oxidation resistance and spark wear resistance as well as peeling resistance may be improved. In particular, a tip formed from an alloy, among Pt—Rh-based alloys, in which the content of elements other than Pt and Rh is less than 5% by mass is particularly excellent in oxidation resistance. However, the alloy in which the content of elements other than Pt and Rh is less than 5% by mass is softer than an Ir alloy, a Pt—Ir-based alloy, a Pt—Rh-based alloy that contains 5% by mass or more of elements other than Pt and Rh, and the like, and it is found that a problem arises that, when a tip having a flange portion is welded to an electrode by resistance welding, the tip is likely to be deformed, and the dimension such as the height of the welded tip is not stabilized. That is, the tip, having a flange portion, formed from a Pt—Rh-based alloy in which the content of elements other than Pt and Rh is less than 5% by mass, is excellent in oxidation resistance and spark wear resistance, while the tips are likely to vary in quality as a product, thereby reducing a yield. Further, also when the tip is held and welded by laser welding without performing resistance welding, there is much concern that the dimension of the welded tip is less likely to be stabilized for the same reason.
Further, Japanese Patent Application Laid-Open (kokai) No. 2005-158322 discloses that resistance welding between a precious metal tip, and a ground electrode or a center electrode is performed to form a flange portion at the bottom portion of the precious metal tip by expansion of the outer diameter of the precious metal tip, whereby peeling of the precious metal tip from the ground electrode or the center electrode is inhibited (see claims 1 and 2, and paragraph 0006 of JP 2005-158322). In example 1 of JP 2005-158322, a precious metal tip formed from a platinum-rhodium alloy is used. However, a platinum-rhodium alloy is soft, and is likely to be deformed. Therefore, a problem may arise that the dimension of the precious metal tip having been welded by resistance welding is not stabilized.
Further, in order to improve wear resistance, the diameter of the tip tends to be increased. An amount of heat required for resistance welding for assuring joining strength is increased according to increase of the diameter of the tip. Therefore, in a case where a tip for which the Pt—Rh-based alloy is used for improving oxidation resistance and spark wear resistance, and which has the flange portion disclosed in JP 4015808 or JP 2008-34393, or has the flange portion formed by resistance welding as disclosed in JP 2005-158322 for assuring peeling resistance, is used, since an amount of heat at resistance welding is increased according to increase of the diameter of the tip, so that a problem arises that the dimension of the tip having been welded by resistance welding is less likely to be stabilized. Further, for example, a problem may arise that spatters and welding droop are generated at the resistance welding, whereby the quality is not stabilized, and a yield is significantly reduced.
Further, for the tip having the flange portion disclosed in JP 4015808 and JP 2008-34393, when the thickness of the flange portion is small, resistance welding needs to be performed while the flange portion is held in order to reduce deformation of the flange portion. At this time, the welding electrode contacts with a portion near a welding surface which is heated, whereby the welding electrode is overheated and the life span of a tool such as the welding electrode is significantly reduced. As a result, for the tip having the flange portion, not only the processing cost is high, but also cost is increased also in production process. Also for the tip having the flange portion formed by resistance welding as disclosed in JP 2005-158322, a great amount of heat is required in order to deform the tip. Therefore, the life span of the tool such as the welding electrode is significantly reduced, resulting in increase of cost.
The present invention is made in view of the above-mentioned problem. Specifically, an object of the present invention is to provide, at low cost, an electrode tip that has oxidation resistance, spark wear resistance, and peeling resistance, and that is less likely to be deformed when welded to an electrode, and a spark plug having the electrode tip.